The invention relates to a process for regenerating and detoxifying electrolytes which contain sodium nitrate and are used in the electrochemical treatment of metallic workpieces, wherein the reaction products formed in the treatment which interfere with the operating process and in some instances are toxic are removed as harmless substances.
In the course of the electrochemical treatment of metals by means of electrolytes based on sodium nitrate, undesirable constituents pass as reaction products into the process solution. These constituents partly come from the materials treated, but partly they are also formed by electrode reactions. For instance, cathodic reduction of the sodium nitrate of the electrolyte gives rise to sodium nitrite. If the metal to be treated contains chronium, sodium chromate is formed by anodic oxidation. Ammonia is formed by cathodic reduction of the nitrite, which had been formed in a similar electrode reaction from the nitrate. Metal hydroxides, essentially iron(III) hydroxide and nickel hydroxide, both pass into the electrolyte through the anodic removal of the material.
All the foreign substances mentioned interfere with the operating process and in some instances are toxic. For instance, chromate reduces the treatment rate wtih increase in concentration until completely seized. The action of nitrite on the operating process is likewise negative on the operating process. In addition, nitrite is toxic and by law must not be allowed to pass into the waste water. The metal hydrolysates from the workpieces interfere very considerably with the treatment process from certain concentrations onward, by being able to form deposits on the edges of tools and workpieces and to lead to reductive residues on the cathodes, thereby producing short circuits which result in the destruction of the expensive tools. The ammonia formed by cathodic reduction of the nitrite formed in the same way passes into the air breathed by the operating personnel and can lead to delicate health problems among them.
It is an object of the present invention to provide a process of the type mentioned at the beginning with which it is possible to regenerate the electrolyte without forming harmful substances.
This object is not achieved by the process known from German Pat. No. 3,303,585, wherein the nitrite is oxidized by means of ozone. This process is costly, for one thing, and not workable without environmental pollution or without special precautions on account of the toxic action of ozone. In addition, this publication deals only with the removal of nitrite.